Highly asymmetric ultrafiltration membranes

ABSTRACT

The present invention relates to highly asymmetric polymer membranes for ultrafiltration (UF), and to a simplified method of their manufacture. Disclosed herein are UF membranes with a high degree of asymmetry between the size of pores at the skin face of the membrane and the size of pores at the opposite face of the membrane. Asymmetry ratios of these membranes range from about 20 to about 1000. The porous support layer between the skin face and the opposite face is reticular and is substantially free of macrovoids. These membranes thus have higher flow rates than prior UF membranes. The membranes are cast from polymer dope mixes that are either homogeneous solutions or stable colloidal dispersions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to filtration membranes. Moreparticularly, the invention relates to polymeric ultrafiltrationmembranes with enhanced flow rates, and to a simplified method of theirmanufacture.

2. Background of the Technology

Filtration membranes are useful for numerous applications wherein it isdesirable to purify or separate components of gaseous or liquidmixtures. Membranes may be classified based in part on these uses. Oneclassification scheme lists types of membranes functionally inincreasing order of their size selectivity: gas separation (GS), reverseosmosis (RO), ultrafiltration (UF), and microfiltration (MF).

UF membranes typically are characterized as having a particularmolecular weight cutoff or exclusion. For example, a membrane capable ofretaining or excluding 90% or more of a macromolecule of 100,000 daltonscould be classified as a 100K filter. Likewise, a membrane effective inefficiently excluding or retaining a macromolecule of 10,000 daltons maybe referred to as a 10K filter. Of course, a 10K filter not only retainsmacromolecules having a MW of 10,000 daltons, but also generally retainsany larger macromolecules in a heterogeneous solution with equal orgreater efficiency. This fact accounts for the problem of membraneclogging that is inversely proportional to the size exclusion rating ofthe membrane--those membranes with a smaller exclusion limit retain moremacromolecules in a heterogeneous mix, and tend to clog or foul morerapidly than membranes with a higher MW exclusion rating.

A particular challenge in the manufacture of UF membranes is to create amembrane with pores that are small enough to achieve an effectiveretention of macromolecules, while maintaining an acceptable flow rateof the fluid to be filtered. A membrane's resistance to fluid flow is afunction of the diameter of the smallest, or retentive, pores throughwhich the fluid must pass, and is also a function of the thickness ofthe layer of retentive pores. All other factors being equal, membraneswhose limiting pores are situated in a relatively thin layer will havefaster flow rates than membranes with a thicker layer of limiting pores.

In principle, an optimal membrane would have all of its limiting poresin one layer that is one molecule thick. Clearly, such a membrane wouldbe practically impossible to manufacture or to handle after manufacture.This is especially true in the UF range and below (RO and GS ranges),because the membranes used in these applications often must structurallywithstand relatively high pressures that are applied to accelerate thefiltration process or to provide energy to overcome diffusion kineticsor osmotic forces. The different structural configurations of UFmembranes, namely composite and integrally skinned membranes, representpractical attempts to approach the ideal of a very thin retentive layer.

Composite membranes have a relatively thin layer of retentive poreslaminated to a support structure, which is often of a composition thatis different from that of the retentive layer. The support structurestabilizes the retentive layer for ease of handling and is intended tooffer very little resistance to fluid flux. Composite UF membranes aredisclosed in PCT International Publication No. WO 96/02317. In thispublication, some disadvantages of composite UF membranes are evident.For example, high filtration pressures can cause delamination of thecomposite and subsequent membrane failure. Additionally, many compositesrequire at least a two stage manufacturing process to form sequentiallythe layers of which the composite is made.

The alternative to a composite membrane is an integral membrane. Theterm integral simply means that the membrane is all of one compositionand is prepared in one casting process, although integral membranes mayhave structurally distinct regions or layers within the integral crosssection. Integral UF membranes typically have a retentive skin incontact with a more porous support structure. In such a membrane, themain function of the support structure is to provide thickness to themembrane for ease of handling. In cross section, this kind of membranedisplays a relatively dense skin at one surface, with an abrupttransition to a matrix of much larger pores in the support structure.

A disadvantage of this type of membrane, especially in the UF range, isthat the support structure has numerous large voids known as macrovoids.These are finger-like projections in the support structure thatgenerally do not communicate with the pores in the skin surface. Fluidentering a macrovoid is trapped and cannot be filtered. Macrovoids in amembrane therefore add to the membrane's resistance to fluid flow,leading to undesirably low flow rates without any concomitant benefit ineffectiveness of filtration. Therefore, while a macrovoid-ridden supportstructure may provide mechanical stability to the skin, thisconfiguration does not result in optimal UF membrane performance.

Skinned UF membranes are disclosed in U.S. Pat. No. 4,481,260 to Nohmi.The '260 patent is directed to hollow fiber membranes for UFapplications. Hollow fiber membranes are commonly used for large-scalefiltration operations wherein it is desirable to maximize the filtersurface area for high total throughput. Typically, hollow fibermembranes must be spun from relatively viscous dope mixes with highconcentrations of total solids. The structure of the Nohmi membranes ischaracterized by a skin layer with pore sizes effective for UF, adjacentto a support structure replete with macrovoids. Accordingly, theperformance of these membranes is limited by the presence of macrovoidsin the support structure. Also, the formulations of the Nohmi patent andother hollow fiber membrane disclosures are generally not appropriatefor or applicable to flat sheet membranes, because of the higher dopeviscosity and total solids required for the spinning of hollow fibermembranes as compared to formulations that may be suitable for castingflat sheet membranes.

Because the average pore diameter of the skin layer of a skinnedmembrane is so different from the average pore diameter of amacrovoid-containing support structure, such membranes have been calledasymmetric, or anisotropic. This term is now often broadly applied toany membrane that has cross-sectional gradations in pore diameter,whether the gradations be abrupt or gradual. However, included amongmembranes that can be called asymmetric are very different structuresthat represent extremes of performance.

The highly asymmetric membrane structure pioneered by U.S. Pat. Nos.4,629,563 (Reexamination Certificate No. B1 4,629,563, issued Jun. 3,1997) and 4,774,039 to Wrasidlo, represents a vast improvement over theso-called asymmetric membranes that preceded it. This is because, in theMF range, the Wrasidlo membrane structure is free of macrovoids. Withoutmacrovoids, the dead space within the membrane is significantly reducedif not eliminated, and flux rates are improved over prior MF membranes.In addition, the high degree of asymmetry within the support structureis gradual, rather than abrupt. This allows the support structure to actas a prefilter (or more accurately, as several prefilters of differentsizes), and greatly enhances the life and dirt-holding capacity of themembrane by retaining particles that are much larger than the skin poreswell before they enter the region of the skin layer.

The manufacture of the Wrasidlo MF membranes is based on the propertiesof an unstable dispersion of a membrane dope mix within the binodal orspinodal curves of a phase diagram. In this method, care must be takenwith each casting to assure that the unstable dispersion is within thebinodal or spinodal in order to achieve the desired membrane structure.The dope mix is typically under constant agitation prior to casting toprevent premature phase separation. Despite the prophetic UF membraneexamples in the Wrasidlo patents, our diligent efforts to produce highlyasymmetric, macrovoid-free UF membranes from an unstable solution havebeen unsuccessful. In membranes having skin porosities appropriate forUF applications, there have always been significant macrovoids presentin the support structure, similar to the structures disclosed in theNohmi patent.

Accordingly, it would be highly desirable to prepare UF membranes havinga highly asymmetric structure analogous to the Wrasidlo membranes.Further, it would be beneficial to create such membranes by a methodthat does not require the careful and constant attenuation of a castingdope in an unstable dispersion. Such an advance would produce UFmembranes with enhanced flow rates and dirt-holding capacity, and themanufacture of these membranes would be greatly simplified.

SUMMARY OF THE INVENTION

The present invention provides asymmetric integral polymer membranes forultrafiltration. Structurally, these membranes include a skin face, anopposite face, and a porous support between the skin face and theopposite face. The skin face has numerous skin pores, that have adiameter of sufficient size to permit passage of a liquid therethroughwhile excluding passage of a macromolecule therethrough. The poroussupport consists of a substantially continuous reticular mass defininginterconnecting flow channels communicating with the skin pores. Theflow channels define support pores having diameters generally increasinggradually in size from the skin face to the opposite face, to an averagediameter at the opposite face of at least about 20 times the diameter ofthe skin pores. The porous support is substantially free of voidsmaterially larger in diameter than the gradually increasing supportpores.

The membrane of this aspect of the invention may be cast as a flatsheet, from a homogeneous solution or a colloidal dispersion. Thecasting solution or dispersion contain a hydrophobic polymer, a solvent,and a nonsolvent. In preferred embodiments, the polymer is a sulfonepolymer, such as, for example, polyethersulfone, polyarylsulfone, orpolysulfone. The solvent may be selected from the group consisting ofN-methylpyrrolidone, dimethylformamide, dimethylacetamide, dioxane,dimethylsulfoxide, chloroform, and tetrachloroethane. A preferredsolvent is N-methylpyrrolidone. The nonsolvent may be selected from thegroup consisting of 2-methoxyethanol, propionic acid, t-amyl alcohol,methanol, ethanol, isopropanol, hexanol, heptanol, octanol, acetone,methylethylketone, methylisobutylketone, butyl ether, ethyl acetate amylacetate, glycerol, diethyleneglycol, di(ethyleneglycol)diethylether,di(ethyleneglycol)dibutylether, and water, with propionic acid beingpreferred.

Membranes of the present invention may exclude macromolecules having amolecular weight of between about 150,000 and 300,000 daltons. Inalternative embodiments, the membranes may have pore sizes adapted toexclude macromolecules of molecular weights from about 75,000 to 150,000daltons. Other embodiments provide pore sizes capable of excludingmacromolecules having a molecular weight between about 10,000 and 75,000daltons. Still other membranes may exclude macromolecules having amolecular weight of less than about 10,000 daltons. Membranes of theinvention may have skin pores with an average diameter of less thanabout 0.03 micron, and some may have skin pores with an average diameterof less than about 0.01 micron.

The membrane of this aspect of the invention may have a flow rate ofwater of at least 0.125 cm/(min×psi). In preferred embodiments, themembrane may have a normalized flow rate quotient (ε) less than 12 ×10⁵,based on the formula ε=e/v², wherein (e) is a molecular weight exclusionvalue for the membrane in daltons, and wherein v is a flow rate of waterof the membrane in cm/(min×psi).

A second aspect of the invention is a method of preparing an asymmetricintegral polymer membrane for ultrafiltration, including the steps of:providing a casting dope including a hydrophobic polymer, a solvent, anda nonsolvent, in ratios sufficient to form a homogeneous solution or acolloidal dispersion; casting the dope to form a thin film; coagulatingthe thin film in a quench bath; and recovering an ultrafiltrationmembrane having an asymmetry ratio of at least about 20, the membranebeing substantially free of macrovoids. This method may include theadditional step of exposing the thin film to air for a preselected time.In some embodiments, the preselected time may be less than about onesecond.

The hydrophobic polymer used in the formulation may be between about 10and 20 percent by weight of the casting dope. The nonsolvent may bebetween about 25 and 70 percent by weight of the casting dope. And thesolvent may be between about 20 and 60 percent by weight of the castingdope. In some embodiments of the method, the hydrophobic polymer is asulfone polymer, such as, for example, polyethersulfone,polyarylsulfone, or polysulfone. The solvent of the formulation may beselected from the group consisting of N-methylpyrrolidone,dimethylformamide, dimethylacetamide, dioxane, dimethylsulfoxide,chloroform, and tetrachloroethane, with a preferred solvent beingN-methylpyrrolidone. The nonsolvent for this method may be selected fromthe group consisting of 2-methoxyethanol, propionic acid, t-amylalcohol, methanol, ethanol, isopropanol, hexanol, heptanol, octanol,acetone, methylethylketone, methylisobutylketone, butyl ether, ethylacetate amyl acetate, glycerol, diethyleneglycol,di(ethyleneglycol)diethylether, di(ethyleneglycol)dibutylether, andwater, with propionic acid being preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view by scanning electron micrograph, at500×magnification, of a 100K UF membrane according to the invention.

FIG. 2 is a cross-sectional view by scanning electron micrograph, at500×magnification, of a 10K UF membrane according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a highly asymmetric UF membranes thatare free of macrovoids, relatively easy to cast, and have enhanced flowrates over other commercially available UF membranes. In one embodiment,the highly asymmetric membranes may be made using sulfone polymers, suchas polysulfone, polyarylsulfone, and polyethersulfone. These membranesare asymmetric, having a skin layer disposed adjacent to a reticularasymmetric support layer. The skin layer may have pores of sufficientdiameter to allow passage of a liquid through the membrane whileexcluding macromolecules from passing therethrough. The membranes of theinvention may be rated according to the lowest molecular weightmacromolecules that they efficiently remove from a solution to befiltered. Accordingly, the invention contemplates numerous "MW cutoff"embodiments, including MW cutoffs of: between 150,000 and 300,000daltons; between 75,000 and 150,000 daltons; between 10,000 and 75,000daltons; and less than 10,000 daltons.

In addition, embodiments of the membrane can include a porous supportcomprising an asymmetric region of gradually increasing pore diameterssuch that the average diameter of the pores at the opposite face of themembrane may be from about 20 to about 1000 times the average diameterof the skin pores. The average diameter of pores at the opposite face,divided by the average diameter of pores at the skin face is theasymmetry ratio of the membrane. In preferred embodiments, membranes ofthe invention typically have asymmetry ratios of from about 30, 40, 50,or 100 to 300, 400, or 500.

Advantages of the Membranes of the Invention

One advantage of the membranes discussed herein is that they can beproduced more efficiently than prior art membranes. It has beendiscovered that the membranes of the present invention can be producedwith a simplified process that does not require employing an unstable ormetastable casting dope. With a metastable casting dope such asdescribed in the prior art, the membranes are normally produced withconstant agitation prior to casting. In addition, special membrane filmexposure conditions are often required following casting. Furthermore,it is sometimes necessary to provide other polymers in the castingsolution or to include a heating step to induce the proper phaseseparation prior to the quenching step.

In one embodiment of the present invention, membranes are prepared byproviding a casting dope comprising between about 10% and 20% by weightof a sulfone polymer, between about 20% and 60% by weight of a solventand between about 25% and 70% by weight of a non-solvent. This castingdope is preferably a stable, clear, homogeneous solution or a stablecolloidal dispersion. The casting dope is then cast to form a thin filmand exposed to a gaseous environment, such as air, for between about 0and 5 seconds. The thin film is then coagulated into a stable membraneby placing the thin film in a quench bath having a temperature betweenabout 5° C. and 60° C.

Architecture of the Membranes

Embodiments of polymer membranes made by the methods disclosed hereinhave a skin face, a porous support, and an opposite face. The poroussupport has a reticular structure, rather than the granular structureseen in many prior UF membranes. The porous support has an asymmetricregion of gradually increasing pore diameters. Because of the graduallyincreasing pore diameters in the porous support, and the relativelylarge pore diameters in the opposite face of the membranes of theinvention, these membranes exhibit a substantial degree of asymmetry. Aconvenient method for assessing the asymmetry and pore diameter ofmembranes is through the use of scanning electron microscopy (SEM).FIGS. 1 and 2 are SEM micrographs illustrating the UF membranes of theinvention in cross section. These figures demonstrate that the membranesof the present invention are highly asymmetric and are substantiallyfree of macrovoids. This high degree of asymmetry and macrovoid-freemorphology has not existed in prior UF membranes. Because of thissignificantly improved membrane structure, the membranes of theinvention have flow rates that are superior to any commerciallyavailable UF membranes of similar MW cutoff ratings. The skin pores ofthe membranes may advantageously have an average diameter from about0.003 micron to about 0.03 micron.

"Asymmetric structure," as used herein, refers to a membrane having adegree of asymmetry between the skin pore diameter and the pore diameterat the opposite face of the membrane that is about 10:1 or greater. Inthat regard, the membranes of the present invention have average skinpore diameters of about 0.003 μm or greater, while the pore diameter atthe opposite face of the membrane may be typically at least about 20times the average skin pore diameter. Thus, the ratio of microporouscast surface pore diameter to skin pore diameter is at least about 20:1and in some embodiments is 100:1, or even 1000:1.

The membranes described herein are substantially free of macrovoids,which are voids that are materially larger in size than the graduallyincreasing pore diameters within the support structure. In addition, themembranes typically have a bulk porosity, or void volume, of greaterthan about 60%. The membranes of the present invention also have asubstantially larger dirt-holding capacity in one flow direction thanthe other, due to the asymmetry of membrane. This enhanced dirt-holdingcapacity can greatly lengthen the life of a UF membrane, since anymacromolecules or other particles significantly larger than the MWcutoff rating of the membrane will be retained within the reticularsupport layer, rather than reaching and clogging the minimum pores.

Some UF applications are optimized by orienting the membrane so that theretentive pores are upstream of the opposite face. Such an orientationis desirable, for example, in protein purification protocols, becausethe protein to be purified accumulates on the skin face and is easilyeluted therefrom, rather than being trapped in the reticular supportlayer as would occur if the membrane were in the dirt-trappingorientation. In the "purification" orientation, of course, thedirt-holding capacity that is normally a benefit of highly asymmetricmembranes is not realized, but the beneficial rapid fluid flux throughthe membrane still occurs, although the flow rate may diminish asmolecules accumulate at the skin face.

Casting Solutions

As discussed above, polyethersulfone membranes can be prepared fromstable, clear, homogeneous solutions as well as stable colloidaldispersions. Membranes are generally prepared from a casting dope ordispersion of a sulfone polymer along with a solvent and a nonsolvent.The concentration of the sulfone polymer in the casting dope should below enough to form a substantially all-reticulated structure within theasymmetric support, but still high enough to produce a coherentmembrane. If the polymer concentration is too low, the resultingmembrane will have no coherency and, in the extreme case, only dust willbe formed. If the polymer concentration is too high, then the membranestructure within the asymmetric support will not be substantiallyreticulated, but will contain at least some granulated structure.

Although the appropriate concentration of polymer varies somewhatdepending upon the particular conditions used (e.g., solvents,nonsolvents, quench temperature), the polymer concentration shouldgenerally be from about 10% to about 20%. Typically, the casting dopecontains from about 12% to about 18% polymer but preferably the castingdope includes about 13-17% polymer.

Several suitable polymers are available and can be used withinembodiments of the invention. Other factors can combine with the kindand concentration of the polymer to affect the stability of the dopemix. Such factors include solvents or their mixtures, nonsolvents ortheir mixtures, and casting temperatures. The following are some of thematerials which have been found useful in the practice of thisinvention, but it will be clear to those skilled in the art that manyothers and/or their combinations may also be used.

A particularly preferred polyethersulfone polymer for use in thepresently claimed invention is E-6020P, manufactured by BASFCorporation, Mount Olive, N.J. Useful PES polymers are also is alsoavailable as Radel A-100, A-200, and A-300, manufactured by AMOCOPERFORMANCE PRODUCTS, Roswell, Ga. There is no particular molecularweight range limitation for polyethersulfone polymers which can beutilized. Another preferred polymer is polysulfone Udel-3500, alsomanufactured by AMOCO.

The preparation of membrane casting dopes wherein the casting dopes arestable, clear, homogeneous solutions or stable colloidal dispersionsinvolves well-known principles of polymer solubility. Solvents forpolymers may be categorized as "good" solvents, nonsolvents, and "poor"(or "ideal") solvents. "Good" solvents are those in which theinteractions between the polymer molecules and solvent molecules aregreater than the forces of attraction between one polymer molecule andanother polymer molecule. The reverse is true for nonsolvents. "Ideal"or"poor" solvents are those in which the interactions between thepolymer and solvent are equal to the forces of attraction between onepolymer and another polymer molecule. Good solvents dissolve substantialamounts of polymer and may be miscible with the polymer, whereas poorsolvents may or may not be miscible, depending upon the molecular weightof the polymer and how poor the solvents are.

It has been found that a stable, clear, homogeneous solution or stablecolloidal dispersion can be obtained by first dissolving the polymer ina good solvent such as N-methylpyrrolidone or dimethylformamide,followed by addition titration just prior to or up to first turbidity ofthe solution with an appropriate amount of nonsolvent such as2-methoxyethanol or propionic acid. The casting dopes are either clear,or if dispersions, have an optical density of less than about 0.5 at 420nm with a 2 centimeter light path. Preferably, the casting dopes areclear, homogeneous solutions.

The solution or dispersion of polymers in the casting dopes useful inthe present invention are in a stable liquid or colloidal dispersioncondition. According to the present invention, this means that thestable liquid or colloidal dispersion will not normally separate intotwo or more distinct liquid phases for a period of at least one or moredays at room temperature. The casting dopes of the present invention areoften stable for several weeks to a month or more at room temperature,especially those containing 2-methoxyethanol as a nonsolvent.Non-limiting examples of solvents that may be used to produce membranesaccording to this invention include: N-methylpyrrolidone,dimethylformamide, dimethylacetamide, dioxane, dimethylsulfoxide,chloroform, and tetrachloroethane.

Among the preferred solvents are N-methylpyrrolidone anddimethylformamide. The amount of solvent which may be employed toprepare an asymmetric polyethersulfone membrane is between about 20% andabout 60% by weight of the casting dope. Preferably, the amount ofsolvent used is between about 25% and about 50% by weight of the castingdope. Most preferably, the precise amount of solvent to be used isdetermined by the particular casting dope, including particular polymer,nonsolvent and the other conditions of the method of preparation of theparticular membrane of interest.

Non-limiting examples of suitable nonsolvents include: 2-methoxyethanol,propionic acid, t-amyl alcohol, methanol, ethanol, isopropanol, hexanol,heptanol, octanol, acetone, methylethylketone, methylisobutylketone,butyl ether, ethyl acetate amyl acetate, glycerol, diethyleneglycol,di(ethyleneglycol)diethylether, di(ethyleneglycol)dibutylether, andwater. Exemplary nonsolvents are 2-methoxyethanol and propionic acid.The amount of nonsolvent which may be employed to prepare an asymmetricpolyethersulfone membrane is between about 25% and about 70% by weightof the casting dope. Preferably, the amount of nonsolvent used isbetween about 35% and about 60% by weight of the casting dope. Mostpreferably, the precise amount of nonsolvent to be used is determined bythe particular casting dope, including particular polymer, solvent andthe other conditions of the method of preparation of the particularmembrane of interest.

In addition to the requirement of a stable polymer-solvent-nonsolventcasting dope, the relationship of these components to the quench liquidis also important. The polymer must, of course, be insoluble in thequench liquid, and the solvent should be soluble in the quench liquid(and vice versa). In fact, it is also preferred that the solvent andquench liquid be completely miscible with each other under membraneformation conditions. Water is the generally preferred quench liquid foreconomic and environmental reasons. It is also preferred that thesolvent have a low viscosity. If the solvent has a low viscosity and thesolvent and quench liquid are completely soluble in one another, rapiddiffusion of the quench liquid into the casting dope is assured.

The nonsolvent should have only limited solubility in the quench liquid.In making ultrafilters, it is preferred that the nonsolvent besubstantially nonmiscible with the quench liquid. This limitedsolubility is believed to be effective in increasing the asymmetry ofthe resulting membrane.

The asymmetry of a porous membrane may be measured by the ratio of theaverage pore diameter of its two faces. As noted hereinabove, theasymmetry ratio for the membranes of this invention is from about 20 toabout 10000.

Casting Processes

The simplified method of the present invention to prepare a UFasymmetric polymer membrane, as stated previously, comprises: providinga casting dope comprising between about 10% and 20% by weight of asulfone polymer, between about 20% and 60% by weight of a solvent andbetween about 25% and 70% by weight of a non-solvent, wherein saidcasting dope is a stable, clear, homogeneous solution or stablecolloidal dispersion; casting the dope to form a thin film; exposing thethin film to a gaseous environment, for between about 0, 0.5, or 1second and 5 seconds; coagulating the film in a quench bath having atemperature between about 5° C. and 60° C.; and recovering thecoagulated membrane.

The membranes of the invention can be cast using any conventionalprocedure wherein the casting solution or dispersion is spread in alayer onto a nonporous support from which the membrane later can beseparated after quenching. The membranes can be cast manually by beingpoured, cast, or spread by hand onto a casting surface followed byapplication of a quench liquid onto the casting surface. Alternatively,the membranes may be cast automatically by pouring or otherwise castingthe dope mix onto a moving belt. A preferred moving belt support ispolyethylene coated paper. In casting, particularly in automaticcasting, mechanical spreaders can be used. Mechanical spreaders includespreading knives, a doctor blade, or spray/pressurized systems. Apreferred spreading device is an extrusion die or slot coater, whichcomprises a chamber into which the casting formulation can be introducedand forced out under pressure through a narrow slot. Membranes may becast by means of a doctor blade with a knife gap of typically about 10to about 20 mils, preferably about 12 through about 18 mils, and morepreferably about 14 mils (375 microns). The relationship between theknife gap at casting and the final thickness of the membrane is afunction of the composition and temperature of the dope mix, theduration of exposure to humid air, the relative humidity of the airduring exposure, the temperature of the quench bath and other factors.Membranes typically collapse upon gelling, losing from about 20% toabout 80% of their thickness. Casting solution or dispersiontemperatures of between about 15° C. and 35° C. are typically utilized.

The air exposure time is generally from about 0 seconds to about 5seconds. In other words, an air exposure is often not necessary.Ultrafiltration membranes are typically produced with the method of theinvention excluding an air exposure, or with a very brief air exposure.Increasing exposure time, over this range, tends to increasepermeability of the resulting membrane.

Following casting and exposure to a gaseous environment, the castdispersion or solution is quenched. In a preferred embodiment, quenchingis accomplished by moving the cast membrane on a moving belt into thequenching liquid, such as a water bath. The quenching liquid is mostcommonly water for the reasons stated above. In the bath, the quenchoperation precipitates or coagulates the polymer and can produce a layerof minimum pores having the requisite pore diameters and a supportregion having the characteristic highly asymmetric structure. Theresulting membrane is ordinarily washed free of entrained solvent andmay be dried to expel additional increments of solvent, diluent, andquench liquid, and thus recover the membrane.

The temperature of the quench bath is an important parameter. Ingeneral, the warmer the quench, the more open will be the membrane.Generally, in the manufacture of membranes in accordance with theinvention, a wide temperature range can be utilized in the quenchingstep, ranging from about 5° C. to about 60° C. Preferably, the quenchliquid is water and the quenching temperature is between about 22° C.and about 45° C. The temperature of the quench bath appears to causemarked changes in the pore diameters of the skin of the membrane andalso in its asymmetry. Where higher quench temperatures are utilized,the membranes may possess both larger skin pores and enhanced asymmetry.Conversely, where lower temperatures are utilized, smaller pores formand asymmetry can be reduced.

Membranes are recovered from the quench bath in the conventional mannerinvolving physical removal, washing with deionized water and oven or airdrying.

The membrane of the invention may be 75 to 225 microns thick, or more.Preferably, the thickness of the membrane is about 90 to 200 microns.More preferably, the membrane thickness is about 110 to 150 microns.Typical membrane thicknesses of the present invention are 110 to 175microns. It is well known that several parameters such as polymersolution viscosity, polymer solution flow rate from the casting knife,membrane support belt speed, environmental and quenching conditions, andthe like, affect membrane thickness. These can all be adjustedempirically to achieve the desired membrane. There is no practical limitto the thickness that may be achieved using the formulation of theinvention. Accordingly, preferred embodiments are determined by what isknown to be most useful for present applications.

The overall properties for the asymmetric UF membranes of the presentinvention may also be varied by varying parameters, such as polymerconcentration, solvent and nonsolvent nature and concentration, relativedegree of homogeneity or stability of the casting dope solution,exposure time to a gaseous environment following casting, quenchingliquid and temperature and other variables of the system such as castingprocess, casting knife gap, and the like. Thus, routine experimentationmay be required to optimize properties for each particular system.

Separation Applications of the Membranes of the Invention

The membranes of the invention are useful for, and exceptionallyefficient in, separation and testing applications. The membranes of theinvention can be utilized in a variety of pharmaceutical applications,such as for pyrogen removal from fluids. The membranes of the inventionare also efficient in a variety of biotechnology applications such asseparation media and in bioreactors and analytical testing applications.Further, the membranes of the invention are highly effective in thefiltration of water. A particularly useful application is the productionof ultrapure water for the electronic industry.

The membranes of the present invention may be used with either the skinside or the support side of the membrane upstream with respect to fluidflow, depending on the application. In the skin-downstream orientation,the reticulated porous support serves as a built-in prefilter, greatlyincreasing the dirt-holding capacity of the membrane. The fluidencounters the largest pores first and later encounters pores havinggradually decreasing size with the smallest pores-those in theskin-being encountered last. Hence larger particles are retained beforethey reach the skin and do not clog its pores. Thus, a graded filtrationis obtained in which the particles are retained at various levelsleaving many more pores available for flow than if they were allretained in one plane at the skin. If the membrane is not highlyasymmetric this advantage does not exist since approximately the sameamount of retained matter fouls both sides of the membrane because thepore diameters on both sides are approximately the same. In applicationswherein it is desirable to purify or concentrate a macromolecule that isin a liquid sample, it is advantageous to place the membrane in the"skin-upstream" orientation, so that the macromolecule to be purifieddoes not become trapped in the membrane.

The membranes of the present invention can be further post treated toimprove their hydrophilicity and the resulting membranes can be utilizedin food, pharmaceutical, biotechnology and water filtration applicationsas described above.

The following examples are provided by way of illustration only and arenot intended as a limitation of the present invention, many variationsof which are possible without departing from the spirit and scopethereof. All parts and percentages in the examples and claims are byweight unless otherwise specified.

EXAMPLES 1-8

In each of the following examples, a membrane dope mix was preparedusing a sulfone polymer, a nonsolvent, and a solvent. For Examples 1-8,the sulfone polymer was polyethersulfone E-6020P, Radel A-100, RadelA-200, or Radel A-300 (all PES); or posulfone Udell-3500 (PSU).Nonsolvents were propionic acid (PA) or 2-methoxyethanol (2-ME). Thesolvent for these Examples was N-methylpyrrolidone (NMP). Membranesamples were cast onto a moving belt of polyethylene coated paper usinga casting knife with a knife gap of 14 or 17 mils. Following casting,the membranes were exposed to air for less than a second and werequenched in a water bath at a temperature specified for each Example.

After coagulation, the membranes were washed with deionized water andthen air dried. The recovered membranes had thicknesses of 110-175microns. Each membrane was tested for water permeability and proteinretention at 10 psig and/or 55 psig on a 25 mm diameter disk Amiconstirred cell, which exposes approximately 3.8 cm² of filter surface fortesting. Also, each membrane of Examples 1-8 was visualized in crosssection via SEM, and each micrograph showed a highly asymmetricstructure similar to those depicted in FIGS. 1 and 2. In addition, eachmembrane tested was assigned an approximate MW cutoff value based onretention efficiency (R %) of protein standards having known molecularweights. Protein standards were immunoglobulin G (IgG, MW=160,000);bovine serum albumin (BSA, MW=67,000); and chymotrypsin (Ctr,MW=24,000).

The results of Examples 1-8 are listed below in Table 1.

                                      TABLE 1    __________________________________________________________________________                      Flow              MW          Non-    Quench                      Rate Pressure                               IgG                                  BSA                                     Ctr                                        Cutoff    Polymer          solvent              Solvent                  Temp                      (ml/min)                           (psi)                               R %                                  R %                                     R %                                        (daltons)    __________________________________________________________________________    1 PES PA  NMP 25° C.                      25   10  96.4                                  -- -- 100K        13.3%            40.6%                46.1%    2 PES 2-ME              NMP 45° C.                      15   10  90.3                                  -- -- 100K      15% 57% 28%    3 PSU PA  NMP 45° C.                      21   10  75 -- -- 200K        13.8%          39%   47.2%    4 PES PA  NMP 22° C.                        6.3                           10  99.7                                  89.4                                     41 30K      15%   36.4%                48.6%    5 PES PA  NMP 25° C.                      5    10  96 90.2                                     10K      15% 38% 47%    5 PES PA  NMP 25° C.                      40   55  97 90 46 10K      15% 38% 47%    6 PES PA  NMP 25° C.                        2.4                           55  99.9                                  94 90 <10K      17%   35.5%                47.5%    7 PES PA  NMP 27° C.                      37   10  99.9                                  99.3                                     75 10K      15% 36% 49%    8 PES PA  NMP 27° C.                           10           10K      15% 38% 47%    8 PES PA  NMP 27° C.                           55           10K      15% 38% 47%    __________________________________________________________________________

Examples 1-8 demonstrate that the properties of the membranes of theinvention are dependent on the interactions of several factors,including the type and concentration of the polymer, nonsolvent, andsolvent. The quench temperature is also important to the finalproperties of the membrane.

Effect of Polymer Concentration

Of the membranes whose properties are listed in Table 1, Example 1,having a MW cutoff of 100K has the lowest polymer concentration, whileExample 6, having a MW cutoff of <10K has the highest concentration ofpolymer. Both membranes were quenched at the same temperature. Althoughthe concentrations of solvent and nonsolvent are not identical for thesetwo membranes, a comparison of Examples 1 and 6 support the generalprinciple that, other factors being held constant, a higher polymerconcentration leads to a membrane with smaller pores at the skinsurface, which results in a membrane with a lower molecular weightcutoff value.

Choice of Nonsolvent

A comparison of Examples 1 and 2 is instructive as it regards theproperties of different nonsolvents in preparation of membranesaccording to the invention. Both membranes were cast from dopes withinthe "stable" range, the dopes being either clear and homogeneous orstable colloidal dispersions. The membrane of Example 1 was cast from adope mix containing 40.6% PA, while the membrane of Example 2 was castfrom a dope mix containing 57% 2-ME. It is evident from these examplesthat the concentrations of components in the dope mix may varysignificantly, depending on which nonsolvent and/or solvent areselected. However, because of the clear distinction between a stable andan unstable dope mix, it is within the ability of a person of ordinaryskill in the art, guided by this disclosure, to select a desired polymerconcentration and then determine proper ratios for the appropriatesolvent and nonsolvent, to achieve a stable dope mix that is either aclear, homogeneous solution or a colloidal dispersion.

Quench Temperature

As mentioned above, the quench temperature has a significant effect onthe morphology of the membrane. A comparison of the membranes ofExamples 1 and 3 is instructive. Both membranes were cast from dopemixes that had identical components and similar formulation ratios ofpolymer, nonsolvent, and solvent. The most marked difference in the waythese membranes were prepared is in the quench temperature, which was25° C. for Example 1 (MW cutoff=100K) and 45° C. for Example 3 (MWcutoff =200K). This comparison is consistent with the general rule that,other factors being held constant, a higher quench temperature resultsin more open pores, and a larger MW cutoff value.

Flow Rates

The Examples listed in Table 1 demonstrate the high flow rates of themembranes of the present invention. It is evident that membranes withlarger limiting pores, and higher MW cutoff values, have a higher flowrate than membranes with more restrictive pore sizes. Based on theprotein retention data, the membrane of Example 6 is classified ashaving a MW cutoff of less than 10K. Its relatively low flow rate isconsistent with the data that indicate that this membrane has smallerlimiting pores than the other membranes of Table 1.

By way of comparison, the membranes of the Nohmi patent, which patentpurports to have superior flow rates over its prior art, have flow ratesthat are much less than those of the present invention. For example, ofthe several 95K membranes disclosed in the Nohmi patent, the one withthe highest flow rate was Nohmi's Example 20, with a flow rate of 33cubic meters per day per square meter of membrane area, at oneatmosphere of pressure. This converts to a flow rate of 0.153centimeters per minute per psi. The flow rates of Examples 1 and 2, both100K membranes, likewise converted to the same units, are 0.658 and0.395 respectively. Although some of this difference arguably could beattributed to differences in MW cutoff classification criteria, even themuch more selective 30K membrane of the present invention (Example 4)has a higher flow rate than the best 95K Nohmi membrane.

Similar comparisons give similar results across all MW cutoff rangesbetween the Nohmi patent and the membranes of the present invention.Likewise, our comparison of flow rates with other commercially availableflat sheet UF membranes shows that the membranes of the presentinvention have much faster flow rates than other UF membranes of similarMW retention grades. The improved flow rates of the present inventionare believed to be due to the highly asymmetric structure of themembranes, and to the absence of macrovoids in these membranes.

Flow rates of UF membranes having different MW cutoffs can be normalizedfor comparison purposes using the formula ε=e/v², where e is thenormalized flow rate quotient, e is the MW exclusion value in daltons,and v is the flow rate in cm/(min×psi). By this formula, UF membranes ofdifferent MW cutoffs can be compared with respect to flow rates. We havefound that the membranes of the invention consistently give ε valuesless than 12×10⁵, while other membranes we have tested are well abovethat value. Low ε values are indicative of high normalized flow rates.

EQUIVALENTS

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent invention as would be understood to those in the art asequivalent and the scope and context of the present invention is to beinterpreted as including such equivalents and construed in accordancewith the claims appended hereto.

What is claimed is:
 1. A method of preparing an asymmetric integralpolymer membrane for ultrafiltration, comprising the steps of:providinga casting dope comprising a sulfone polymer, a solvent, and anonsolvent, in ratios sufficient to form a homogeneous solution or acolloidal dispersion; casting said dope to form a thin film; coagulatingsaid thin film in a quench bath; and recovering an ultrafiltrationmembrane having an asymmetry ratio of at least about 20, said membranebeing substantially free of macrovoids.
 2. The method of claim 1,comprising the additional step of exposing said thin film to air for apreselected time.
 3. The method of claim 1, wherein said preselectedtime is less than about one second.
 4. The method of claim 1, whereinsaid hydrophobic polymer is between about 10 and 20 percent by weight ofsaid casting dope.
 5. The method of claim 1, wherein said nonsolvent isbetween about 25 and 70 percent by weight of said casting dope.
 6. Themethod of claim 1, wherein said solvent is between about 20 and 60percent by weight of said casting dope.
 7. The method of claim 1,wherein said sulfone polymer is polyarylsulfone or polysulfone.
 8. Themethod of claim 1, wherein said sulfone polymer is polyethersulfone. 9.The method of claim 1, wherein said solvent is selected from the groupconsisting of N-methylpyrrolidone, dimethylformamide, dimethylacetamide,dioxane, dimethylsulfoxide, chloroform, and tetrachloroethane.
 10. Themethod of claim 9, wherein said solvent is N-methylpyrrolidone.
 11. Themethod of claim 1, wherein said nonsolvent is selected from the groupconsisting of 2-methoxyethanol, propionic acid, t-amyl alcohol,methanol, ethanol, isopropanol, hexanol, heptanol, octanol, acetone,methylethylketone, methylisobutylketone, butyl ether, ethyl acetate amylacetate, glycerol, diethyleneglycol, di(ethyleneglycol)diethylether,di(ethyleneglycol)dibutylether, and water.
 12. The method of claim 11,wherein said nonsolvent is propionic acid.